Re: [RFC][PATCH] x86: make text_poke() atomic

From: Masami Hiramatsu <mhiramat@redhat.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nick Piggin <npiggin@suse.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andi Kleen <andi@firstfloor.org>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Arjan van de Ven <arjan@infradead.org>,
	Rusty Russell <rusty@rustcorp.com.au>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Steven Rostedt <srostedt@redhat.com>
Subject: Re: [RFC][PATCH] x86: make text_poke() atomic
Date: Mon, 02 Mar 2009 19:31:27 -0500	[thread overview]
Message-ID: <49AC7A5F.7080009@redhat.com> (raw)
In-Reply-To: <20090303002214.GA4147@elte.hu>

Ingo Molnar wrote:
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
> 
>>
>> Ingo Molnar wrote:
>>> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>>>
>>>> Ingo Molnar wrote:
>>>>>>> So perhaps another approach to (re-)consider would be to go back 
>>>>>>> to atomic fixmaps here. It spends 3 slots but that's no big 
>>>>>>> deal.
>>>>>> Oh, it's a good idea! fixmaps must make it simpler.
>>>>>>
>>>>>>> In exchange it will be conceptually simpler, and will also scale 
>>>>>>> much better than a global spinlock. What do you think?
>>>>>> I think even if I use fixmaps, we have to use a spinlock to protect
>>>>>> the fixmap area from other threads...
>>>>> that's why i suggested to use an atomic-kmap, not a fixmap.
>>>> Even if the mapping is atomic, text_poke() has to protect pte
>>>> from other text_poke()s while changing code.
>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
>>> Well, but text_poke() is not a serializing API to begin with. 
>>> It's normally used in code patching sequences when we 'know' 
>>> that there cannot be similar parallel activities. The kprobes 
>>> usage of text_poke() looks unsafe - and that needs to be fixed.
>> Oh, kprobes already prohibited parallel arming/disarming
>> by using kprobe_mutex. :-)
> 
> yeah, but still the API is somewhat unsafe.

Yeah, kprobe_mutex protects text_poke from other kprobes, but
not from other text_poke() users...

> In any case, you also answered your own question:
> 
>>>> Even if the mapping is atomic, text_poke() has to protect pte
>>>> from other text_poke()s while changing code.
>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
> 
> kprobe_mutex does that.

Anyway, text_edit_lock ensures that.

By the way, I think set_fixmap/clear_fixmap seems simpler than
kmap_atomic() variant. Would you think improving kmap_atomic_prot()
is better?

> 
> 	Ingo
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-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com